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ntohl and htonl

Posted on 2009-05-08
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Last Modified: 2012-05-06
In my assignment I am going to send structs over the UDP datagram.. the spec says that everything is in network byte order..

one example of the struct is the following.

So before sending the struct to the other machine, which is a 64 bit machine... what should I do? Should I call htonl?? as I've tried that and the other machine receives the status of the structs differently
typedef struct AARP_Packet {
    u_char			version;		// AARP_VERSION
    u_char			operation;		// See below
    AIP_Addr		aip;		// AIP address
    struct in_addr	ip;			// IP address
    in_port_t		port;		// Port
    u_short			status;			// See below
} AARP_Packet;
 
 
AARP_Packet* temp = malloc(sizeof(AARP_Packet));

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Question by:kuntilanak
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6 Comments
 
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Infinity08 earned 500 total points
ID: 24336096
>> what should I do? Should I call htonl??

Correct.

>> as I've tried that and the other machine receives the status of the structs differently

The other side will need to do ntohl to convert it back from network byte order to host byte order.


A few extra thoughts :

(a) did you make sure that u_short is a 32 bit unsigned integer type on both platforms ? How about all other used types - do they have the same sizes on both platforms ?
(b) did you make sure that there is no struct padding added ?
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Author Comment

by:kuntilanak
ID: 24336178
how can I make sure the u_short is a 32 bit unsigned integer type??

yes, I think the struct is not padded.. AIP_Addr here is just an u_short, it's just another name for it.. so say when I want to send a struct with AIP_Addr of 2, I should do the following:

Is the code below correct?

I've attached a picture on how I laid out the struct


temp->aip = ntohl(2);

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pic.JPG
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by:Infinity08
ID: 24336307
>> how can I make sure the u_short is a 32 bit unsigned integer type??

Did you define it to be a 32bit unsigned integer type ?
How is it defined ?


>> yes, I think the struct is not padded..

If you're not sure, you can't rely on it. The compiler is free to add padding if it thinks that's useful. Some compilers have switches to force it not to add padding.


>> Is the code below correct?

All depends on your struct layout, and the element sizes.


Can you post what the output of this code is for both platforms :
#include <stddef.h>
 
fprintf(stdout, "sizeof(u_char) = %u\n", sizeof(u_char));
fprintf(stdout, "sizeof(AIP_Addr) = %u\n", sizeof(AIP_Addr));
fprintf(stdout, "sizeof(struct in_addr) = %u\n", sizeof(struct in_addr));
fprintf(stdout, "sizeof(in_port_t) = %u\n", sizeof(in_port_t));
fprintf(stdout, "sizeof(u_short) = %u\n", sizeof(u_short));
fprintf(stdout, "sizeof(AARP_Packet) = %u\n", sizeof(AARP_Packet));
 
fprintf(stdout, "offsetof(AARP_Packet, version) = %u\n", offsetof(AARP_Packet, version));
fprintf(stdout, "offsetof(AARP_Packet, operation) = %u\n", offsetof(AARP_Packet, operation));
fprintf(stdout, "offsetof(AARP_Packet, aip) = %u\n", offsetof(AARP_Packet, aip));
fprintf(stdout, "offsetof(AARP_Packet, ip) = %u\n", offsetof(AARP_Packet, ip));
fprintf(stdout, "offsetof(AARP_Packet, port) = %u\n", offsetof(AARP_Packet, port));
fprintf(stdout, "offsetof(AARP_Packet, status) = %u\n", offsetof(AARP_Packet, status));

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Author Comment

by:kuntilanak
ID: 24336357
This is what I got on the 32 bit machine and I can't test it on the 64 bit machine, but it can always be assumed to parse it correctly as it's given as a part of the assignment in binary...
sizeof(u_char) = 1
sizeof(AIP_Addr) = 2
sizeof(struct in_addr) = 4
sizeof(in_port_t) = 2
sizeof(u_short) = 2
sizeof(AARP_Packet) = 12

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Expert Comment

by:Infinity08
ID: 24336423
>> but it can always be assumed to parse it correctly as it's given as a part of the assignment in binary...

Ok.

Then you'll need to show a bit more code :)
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Expert Comment

by:evilrix
ID: 24338401
Just as an observation, which I'll let I8 elaborate on if he so chooses (and this isn't mean to contridict anything said so far), generally the safest way to send compound objects across heterogeneous networks is to serialise them first. This removes doubt over things such as packing or data type sizes. It is; however, more work since you need to define a serialization protocol.
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